Urban Emission Sources

Blumenthal and co-workers used the earlier data and a case study of Denver, Colorado, to develop arguments as to the source of high concentrations of ozone or ozone precursors that are found in some nonurban areas. Th believe that downwind areas as long as 260 km can exceed the standard because of precursor emission from an urban source. Although th do not present any ozone production estimates related to photochemistry, their data analyses confirm hypotheses of transport from urban to nonurban areas. Th point out that rural areas have had concentrations as high as 0.3 ppm wi no local source of reactant. 

Hildemann, L. M., Markowski, G. R., and Cass, G. R. (1991). Chemical composition of emissions from urban sources of fine organic aerosol. Environ. Sci. Technol. 25, 744-759. 

The urban heat island effect has implications for chemical emissions in several ways. Elevated temperatures increase the consumption of energy for air conditioning and other cooling systems, with attendant increases in fossil fuel combustion-related contaminants (Cardelino et al., 2001 Adams, 1999). Increased ambient air temperatures also increase volatilization of POPs such as PCBs, PCNs and PBDEs from urban sources (Priemer and Diamond, 2002 Helm and Bidleman, 2003). 

Nonpoint sources of pollution are more difficult to measure because they often cover large areas or are a composite of numerous point sources. Examples of nonpoint sources include pesticide and fertilizer runoff from agricultural fields, and urban runoff contaminated with pollutants from automobile emissions. Nonpoint sources may not be directly located next to a surface water body pollutants may be transported to surface waters by runoff from the land, by groundwater inflow, or by atmospheric transport. 

For the other metals, Hg correlates with Zn, Pb and Cd at SC but not at CBL. Similarly, Hg did not correlate with other metals in Florida (30). The metals Cu, Ni and Cr are strongly correlated at both sites. Mercury, which is transported globally, also likely has a two component signal from local sources -coal combustion emissions and waste (medical and municipal) incineration while Pb and Cd have their primary sources as waste incineration (P, II). The overall inter-relationships of Pb, Cd and Hg suggest that overall, in the city, Pb and Cd do have different sources. The metals Pb, Cd, Cu and Zn are well correlated at CBL (Table III). This suggests that the correlations found for Hg at SC reflect the urban sources and not the regional picture. 

Human-made sources cover a wide spectrum of chemical and physical activities and are the major contributors to urban air pollution. Air pollutants in the United States pour out from over 10 million vehicles, the refuse of over 250 million people, the generation of billions of kilowatts of electricity, and the production of innumerable products demanded by eveiyday living. Hundreds of millions of tons of air pollutants are generated annu ly in the United States alone. The five main classes of pollutants are particulates, sulfur dioxide, nitrogen oxides, volatile organic compounds, and carbon monoxide. Total emissions in the United States are summarized by source categoiy for the year 1993 in Table 25-10. 

The behavior of these pollution roses is intuitively plausible, because considerable hydrocarbon emissions come from motor vehicles which are operated in both winter and summer and travel throughout the urban area. On the other hand, sulfur dioxide is released largely from the burning of coal and fuel oil. Space heating emissions are high in winter and low in summer. The SO2 emissions in summer are probably due to only a few point sources, such as power plants, and result in low average concentrations from each direction as well as large directional variability. 

The most widespread and persistent urban pollution problem is ozone. The causes of this and the lesser problem of CO and PMjq pollution in our urban areas are largely due to the diversity and number of urban air pollution sources. One component of urban smog, hydrocarbons, comes from automobile emissions, petroleum refineries, chemical plants, dry cleaners, gasoline stations, house painting, and printing shops. Another key component, nitrogen oxides, comes from the combustion of fuel for transportation, utilities, and industries. 

In the United States, in particular, recent legislation has mandated sweeping improvements to urban air quality by limiting mobile source emissions and by promoting cleaner fuels. The new laws require commercial and government fleets to purchase a substantial number of vehicles powered by an alternative fuel, such as natural gas, propane, electricity, methanol or ethanol. However, natural gas is usually preferred because of its lower cost and lower emissions compared with the other available alternative gas or liquid fuels. Even when compared with electricity, it has been shown that the full fuel cycle emissions, including those from production, conversion, and transportation of the fuel, are lower for an NGV [2]. Natural gas vehicles offer other advantages as well. Where natural gas is abundantly available as a domestic resource, increased use... 

An inventory of SO2 emissions has been conducted in an urban area by square areas, 5000 feet (1524 meters) on a side. The emissions from one such area are estimated to be 6 g/s for lire entire area. This square is composed of residences and a few small commercial establislunents. What is lire concentration resulting from tliis area at tlie center of tlie adjacent square to tire nortlt when tlie wind at 2.5 m/s The average effective stack height of tliese sources is assumed to be 20 meters. 

Most urban rail service is electric-powered and most urban bus service is diesel-powered, although diesel rail and electric bus operations do exist, as noted above. The efficiency and environmental impacts of electricity depend gi eatly on the source of electric power. Although electric vehicles produce no tailpipe emissions, generation of electricity can produce significant emissions that can travel long distances, Eor example, coal-powered electricity plants produce particulate emissions that travel halfway across North America, Urban buses also can be powered by a variety of alternative fuels. 

Air pollution is principally a problem in urban and heavily industrialized areas, where the flow of clean air from surrounding areas is insufficient to disperse the accumulation. Motor vehicles account for more than 50% of the man-made emissions of carbon monoxide, hydrocarbons, and nitrogen oxides (4). More than half of the U.S. annual trillion vehicle miles are driven in urban areas (5). Nature produces much more pollutants than all man-made sources, but natural emissions are widely dispersed and do not contribute heavily to urban pollution problems (6, 7). 

Diverse techniques have been employed to identify the sources of elements in atmospheric dust (and surface dust) (Table V). Some involve considering trends in concentration and others use various statistical methods. The degree of sophistication and detail obtained from the analyses increases from top left to bottom right of the Table. The sources identified as contributing the elements in rural and urban atmospheric dusts are detailed in Table VI. The principal sources are crustal material, soil, coal and oil combustion emissions, incinerated refuse emissions, motor vehicle emissions, marine spray, cement and concrete weathering, mining and metal working emissions. Many elements occur in more than one source, and they are classified in the... 

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